RLSA antenna having two orthogonal linear polarizations

ABSTRACT

The network antenna of the RLSA type in the form of a radial waveguide has a feed structure allowing simultaneous excitation of the antenna in two orthogonal linear polarizations. The feed structure, essentially placed to the rear of the antenna, consists of a circular waveguide placed at the centre of the radial waveguide and coupled to the latter by two circular slots for the excitation of the antenna in a first linear polarization and of a coaxial waveguide surrounding the circular waveguide and coupled to the radial waveguide by radial slots, the coaxial waveguide being excited by a ring-shaped waveguide placed coaxially on the outer periphery of the coaxial waveguide and coupled to the latter by slots distributed around the inner periphery of the ring for the excitation of the antenna in a second linear polarization orthogonal to the first linear polarization.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP03/150357, filed Aug. 4, 2003, which waspublished in accordance with PCT Article 21(2) on Mar. 4, 2004 inEnglish and which claims the benefit of France patent application No.0210507, filed Aug. 23, 2002.

BACKGROUND OF THE INVENTION

The invention relates to network antennas of the radial waveguide typewith linear slots, known as RLSAs (Radial Line Slot Antennas), that areintended to be more particularly used in satellite communicationssystems. In these communications systems, transmission to the satelliteand reception from the satellite take place along two orthogonal(circular or linear) polarizations, respectively, although thetransmission and reception frequency bands are generally different. Thisdecoupling between the two links—the uplink and the downlink—of thecommunications system is enhanced the better the isolation between thetwo orthogonal polarizations in the network antenna. The identicalapproach of two orthogonal polarizations is used in wireless terrestrialcommunications systems, known as LMDSs (Local Multipoint DistributionSystems), that operate in the 40 GHz millimetric bands.

A network antenna of the RLSA type having a feed structure that allowsthe antenna to be excited in two orthogonal linear polarizations isknown from the document by F. J. Boebels & K. C. Kelly entitled“Arbitrary Polarization From Annular Slot Planar Antennas” published inIRE TRANSACTIONS ON ANTENNAS AND PROPAGATION, July 1961, pages 342–349.The feed structure for this antenna consists of two radial cavities, oneplaced above the other inside the antenna, each cavity being excited bya circular waveguide placed at the centre of one of the two faces of theantenna. The two ports of the feed structure are thus placed on eitherside of the antenna, this having the effect of creating masking andperturbation regions at the front of the antenna and thereforedowngrading the radiation characteristics of the latter.

SUMMARY OF THE INVENTION

The object of the invention is to remedy this drawback and for thispurpose a network antenna of the RLSA type in the form of a radialwaveguide according to the invention is characterized in that the feedstructure, essentially placed to the rear of the antenna, consists of acircular waveguide placed at the centre of the radial waveguide andcoupled to the latter by two circular slots for the excitation of theantenna in a first linear polarization and of a coaxial waveguidesurrounding the circular waveguide and coupled to the radial waveguideby radial slots, the coaxial waveguide being excited by a ring-shapedwaveguide placed coaxially on the outer periphery of the coaxialwaveguide and coupled to the latter by slots distributed around theinner periphery of the ring for the excitation of the antenna in asecond linear polarization orthogonal to the first linear polarization.

According to the features of the network antenna according to theinvention:

-   -   the first linear polarization is excited by means of a first        rectangular input waveguide propagating the TE₀₁, fundamental        mode, oriented along an axial direction of the antenna, in the        circular waveguide;    -   the second linear polarization is excited by means of a second        rectangular input waveguide propagating the TE₀₁ fundamental        mode, oriented in a direction perpendicular to the axial        direction of the antenna, in the ring-shaped waveguide;    -   the two rectangular input waveguides are placed parallel to each        other;    -   the two rectangular input waveguides are placed one beneath the        other.

With this construction of the feed structure for the antenna, the firstrectangular waveguide may be above or below the second rectangularwaveguide and various topologies may be envisaged for the arrangement ofthe electronic cards for transmitting/receiving the microwave signals.Moreover, the construction of the feed structure based on imbricatedwaveguides is relatively simple to produce.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of an RLSA network antenna according to the invention isdescribed below and illustrated in the drawings.

FIG. 1 shows the network antenna with a feed structure according to theinvention, the unit being seen in axial section of the antenna.

FIG. 2 is a partial perspective view in axial section of the antenna.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, the RLSA-type network antenna comprises two coaxial circularconducting plates 1, 2 that may be separated from each other by adielectric material in order to form a radial waveguide 3. FIG. 2 showsthe front face 1 of the antenna, which has an array of discreteradiating slots F arranged in concentric circles. It is obvious to thoseskilled in the art that the radiating slots may have anotherarrangement.

The feed structure for the network antenna 3 is essentially placedadjacent to the rear face 2 of the antenna such that its front face isnot masked by an element that disturbs its radiation pattern.

The feed structure is designed to simultaneously excite, in transmissionand/or reception, the two modes E₁₁ and H₁₁ by means of a pair ofrectangular input waveguides 4, 5 that are placed at the rear of theantenna and extend, in the embodiment illustrated, perpendicular to thecentral axis A of the antenna. These two input waveguides 4, 5 may beplaced parallel to each other and with one below the other in order toconstitute two independent excitation ports. However, other arrangementsmay be adopted without departing from the scope of the invention.

To excite the first mode E₁₁, the first input waveguide 4 is coupled tothe radial waveguide 3 via a circular waveguide 6 that is placed at thecentre of the rear face 2 of the antenna. The input waveguide 4 is fedwith its TE₀₁ fundamental mode, the electric field of which, indicatedby an arrow in FIG. 1, is parallel to the direction A. The circularwaveguide 6 propagates the TM₀₁ mode into the radial waveguide 3 via twocircular coupling slots 7, 8 spaced apart inside the radial waveguide 3along the axis A by a half-wavelength of the guided wave in the TM₀₁mode.

To excite the second mode H₁₁, the second input waveguide 5 is coupledto the radial waveguide 3 via a ring-shaped waveguide 9 that is itselfcoupled to a coaxial waveguide 10 surrounding the circular waveguide 6,this coaxial waveguide being coupled to the radial waveguide 3. Thering-shaped waveguide 9 is placed coaxially on the outer periphery ofthe coaxial waveguide 10. The input waveguide 5 is fed with its TE₀₁fundamental mode, the electric field of which, indicated in FIG. 1 by anarrow (perpendicular to the plane of the sheet), is perpendicular to thedirection A. The TE₀₁ mode is propagated by the ring-shaped waveguide 9into the coaxial waveguide 10 via linear coupling slots 11 distributedaround the inner periphery of the ring, while the coaxial waveguide 10is coupled to the radial waveguide 3 via radial coupling slots 12fanning out from the rear face 2 of the antenna around the circularwaveguide 6. The mean circumference of the ring-shaped waveguide 9 is amultiple of the wavelength of the guided wave in the TE₀₁ mode. In theembodiment illustrated, the coaxial cavity 10 is coupled via 12 radialstops 12 to the radial waveguide 3 and the mean inside circumference ofthe waveguide 9 is equal to twelve times the wavelength of the guidedwave in the TE₀₁ mode.

Of course, the size of the coupling slots 11 and 12 and theirdistribution should be adjusted in order to obtain the desiredperformance of the antenna in terms of bandwidth, efficiency andmatching.

This feed structure therefore makes it possible, from the rear face ofthe antenna, to excite the latter in two orthogonal linear polarizationswhile avoiding the presence of masking and perturbation regions at thefront of the antenna.

1. Network antenna of the RLSA type in the form of a radial waveguideand having a feed structure allowing simultaneous excitation of theantenna in two orthogonal linear polarizations, wherein the feedstructure, essentially placed to the rear of the antenna, consists of acircular waveguide placed at the centre of the radial waveguide andcoupled to the latter by two circular slots for the excitation of theantenna in a first linear polarization and of a coaxial waveguidesurrounding the circular waveguide and coupled to the radial waveguideby radial slots, the coaxial waveguide being excited by a ring-shapedwaveguide placed coaxially on the outer periphery of the coaxialwaveguide and coupled to the latter by slots distributed around theinner periphery of the ring for the excitation of the antenna in asecond linear polarization orthogonal to the first linear polarization.2. Antenna according to claim 1, in which the first linear polarizationis excited by means of a first rectangular input waveguide propagatingthe TE₀₁ fundamental mode, oriented along an axial direction of theantenna, in the circular waveguide.
 3. Antenna according to claim 2, inwhich the second linear polarization is excited by means of a secondrectangular input waveguide propagating the TE₀₁, fundamental mode,oriented in a direction perpendicular to the axial direction of theantenna, in the ring-shaped waveguide.
 4. Antenna according to claim 3in which the first and second rectangular input waveguides are placedparallel to each other.
 5. Antenna according to claim 4, in which thefirst and second rectangular input waveguides are placed one beneath theother.